1. Field of the Invention
The invention relates to modified tie-layer adhesive compositions and to their use in coextruded multi-layer barrier films. The adhesive blends which area utilized as the tie-layer to adhere the barrier layer and polyolefin layer(s) are blends of a polyethylene resin, graft-modified, i.e., functionalized polyethylene and saturated bicyclic modifier compound.
2. Description of Prior Art
Multi-layer barrier films are widely used in the packaging industry primarily for food packaging applications where film clarity is an important consideration. Such applications include wraps for meat and cheese, formed bags and pouches including cook-in packaging. They are also used for packaging snack foods, cereals, baking mixes and the like.
Depending on the intended end-use application of the multi-layer barrier film, the number and arrangement of the resin layers and the types of resins employed will vary. Polyethylene (PE) resins are widely used based on their ability to provide essential properties such as structural integrity, puncture resistance, abrasion resistance, etc. Ethylene-vinyl alcohol (EVOH) copolymers and polyamides (PA) are widely used as a barrier layer to prevent the passage of oxygen and moisture.
To facilitate adhesion of the non-polar polyethylene layer(s) and polar barrier resin layer(s), adhesive layers referred to as tie-layers, are employed. Tie-layers are generally mixtures of functionalized polyolefins, i.e., polyolefins having polar functionality grafted onto the polymer backbone, and non-functionalized polyolefins. While various tie-resin compositions are available and known to effectively adhere PE to EVOH or PA this is often accompanied by an undesirable decrease in film clarity and/or grainy appearance—the latter often referred to as “orange peel.” Even though the causes of these visual phenomena are not completely understood, the problems are generally believed to be the result of interfacial distortion caused by reaction of the functionality of the tie resin with the functional groups of the EVOH or nylon barrier resin. While the adhesive bond strength is directly proportional to the number of bonds formed at the tie-EVOH (or PA) interface, this same bonding effectively amplifies shear stresses caused by viscosity differences between the tie resin and barrier resin. The phenomena can be observed by coextruding linear low density polyethylene (LLDPE) and EVOH film and adding increasing amounts of grafted maleic anhydride (MAH) to the LLDPE. As the amount of the grafted MAH increases, there is a corresponding increase in adhesion and interfacial distortion, i.e., reduction in film clarity. This occurs even though the viscosity of the LLDPE does not significantly change as the grafted MAH is added. The degree of interfacial distortion is determined using Narrow Angle Light Scattering techniques (ASTM D1746-03).
Various techniques have been employed in an effort to improve clarity and eliminate or reduce orange peel in multi-layer barrier films while maintaining high adhesion between the film layers. One such approach is disclosed in copending application Ser. No. 10/842,924 which utilizes linear low density ethylene-α-olefin (LLDPE) copolymers having a network structure as the base resin for the tie-layer adhesive composition. Whereas it is possible to produce barrier films having improved clarity using the tie-layer composition of the invention, the compositions require the use of base resins which are more costly to produce than conventional PE base resins.
Accordingly, it would be highly advantageous if adhesive compositions produced using conventional PE base resins and having improved clarity in multi-layer constructions were available. It would be even more advantageous if this could be accomplished by the addition of small amounts of readily available modifying agents to existing PE-based tie-layer compositions. These and other advantages are achieved with the present invention.
Various fused ring compounds, e.g., disodium bicyclo[2.2.1]heptene dicarboxylate and N-carboxymethyl-bicyclo[2.2.2]oct-5-en-2,3-dicarboximide, are disclosed as nucleating agents primarily for propylene homopolymers and copolymers of propylene with ethylene, 1-butene, 1-hexene, 1-octene, 4-methyl-1-pentene, vinyl acetate and methyl acrylate in U.S. Pat. Nos. 5,929,146 and 5,981,636. Whereas the references disclose use of the nucleating agents for copolymers of ethylene and comonomers containing functional groups, such as acrylic acid, methyl and ethyl acrylate and vinyl acetate, there is no suggestion of their use with graft-modified polymer and particularly with tie-layer adhesive blends of polyethylene resins and graft-modified polyethylene resins. Moreover, the disclosed fused ring nucleating agents are utilized to enhance the properties of molded goods produced from the propylene polymers and there is no disclosure of their use for coextruded multi-layer films and, more particularly, in the tie-layer compositions used for the production of coextruded multi-layer films.
PCT application WO 02/094759 discloses saturated bicyclic compounds which are structurally similar to those disclosed in U.S. Pat. Nos. 5,981,636 and 5,929,146 as nucleating agents for polypropylene to provide high peak crystallization temperatures and other benefits. While the reference discloses that propylene polymer compositions containing the saturated bicyclic nucleating agents can be fabricated into films, the reference is totally silent with regard to their use in tie-layer adhesive compositions and for multi-layer barrier film applications.